Photometabolism of bilirubin will be studied by fluorescent irradiation of bile duct cannulated Gunn rats. The photo products of bilirubin, photo-induced isomers and photo-oxygenation products, will be isolated and quantified. The cholestasis and diarrhea associated with phototherapy for neonatal hyperbili-rubinemia will be quantitated by monitoring serum concentrations of conjugated bilirubin and bile salts. The photometabolic products in serum, urine and stools are to be measured. The diarrhea will be quantitated by gut transit time. The nature of the diarrhea will be studied in intestinal perfusions in the hamster by inclusion of isolated photoproducts of bilirubin in the infusion media and rate of absorption and secretion quantitated. Amino acid conjugates of bilirubin are to be isolated and characterized in humans and other animal species.